UMass prof questions moon origin theory

AMHERST — “Everyone knows” our moon formed when a Mars-sized body crashed into the young Earth and knocked off part of its outer mantle. Even grade school kids learn this giant impact scenario of our moon’s origin. But perhaps it isn’t so, says Donald Wise, professor emeritus of geosciences at the University of Massachusetts.

Writing in the January issue of Physics Today, Wise points out that new models proposed in 2012 assume “starting conditions” similar to those in a long-abandoned lunar origin model he and several others proposed in the 1960s. These conditions explain how the moon formed directly from Earth’s mantle with no need for a giant impact. It’s time, Wise suggests, that “our quest to answer one of mankind’s oldest questions should expand to include this simpler hypothesis.”

Wise says underpinnings of currently accepted “giant impact” lunar origin models are far shakier than generally recognized because they all fail a major test: No evidence of impact contamination by a non-Earth body has ever been found on the moon. He notes that increasingly precise isotopic and chemical analyses developed over the last 40 years show lunar rocks are identical with Earth’s mantle, in some cases to parts per million.

“Even though there is no trace of contamination of lunar rocks by some foreign body of the solar system, the impact model has so permeated our understanding that most people believe the origin of the moon was by giant impact,” he said. “Until two years ago, no computer simulation could explain the composition of real lunar samples.”

In the early 1960s, Wise was one of several geologists who proposed a “fission” origin for the moon. It held that the settling of the dense core of the rapidly spinning early Earth caused acceleration beyond stability, so part of the outer mantle spun off to form the moon. Other competing models said the moon was either a minor partner of an original double planet system or a foreign body “captured” by gravity from elsewhere in the solar system. By decade’s end, the capture hypothesis was widely accepted.

During the lead-up to Apollo 11’s 1969 moon landing, Wise was working in NASA’s Lunar Exploration Office helping to plan the first extra-terrestrial field trip and looking forward to rock samples that would help test the capture and fission hypotheses. In 1969, Wise had hypothesized that extreme heat vaporized part of Earth’s mantle into an incandescent atmosphere. Its escape into outer space took the excess angular momentum with it and slowed the system toward observed modern values.

Then in 1975, the giant impact hypothesis opened a new era of lunar origin research, Wise says. At last, it was a hypothesis capable of being tested by computer simulations. Increasingly detailed analyses showed Earth’s mantle and the moon’s composition are not only similar but for some isotopes, identical. After a third of a century of failed attempts to simulate those compositions with more and more sophisticated giant-impact simulations, “it began to look like this hypothesis had begun to approach a dead end,” he notes.

In his letter this month, Wise upgrades the older model and suggests that a giant impact is an unnecessary complication, because a simpler scenario can use similar conditions to produce the same results. Thus a revitalized version of core-driven mantle spin-off is a robust alternate candidate to explain the moon’s origin. He now suggests, “the scientific community needs to reconsider the hypotheses of lunar origin and its bandwagon should be considering a much richer array of exploration possibilities.”

The angular momentum of an oblique impact of a Mars-sized 'Theia' necessary to form the Moon appears to obviate the leading hypothesis for the formation of 'Theia' in one of Earth's stable, L4 or L5, Lagrangian points.
Calculation of the pre-impact 'offset velocity' of a Mars-sized planet, 'Theia', which grazes Earth's limb (Earth's 6,378 km radius) with a pre-impact 'linear offset momentum' equivalent to the post-impact lunar angular momentum.
Mass of Moon: mMoon = .0123 mEarth
Mass of Mars: mMars = .107 mEarth
Semi-major axis: rMoon = 384,399 km
Average orbital velocity of Moon: vMoon = 1.022 km/s
Radius: rEarth = 6378 km
L1 = L2
(mMars)(rMars)(vMars) = (mMoon)(vMoon)(rMoon)
(.107 mEarth)(6,378 km)(vMars) = (.0123 mEarth)(384,399 km)(1.022 km/s)
vMars = 7.08 km/s
Since low Earth orbit begins at 6.9 km/s (ignoring the barycenter offset of a Mars-sized body as a rounding correction) a (point-sized) Mars-sized body, 'Theia, in low Earth orbit has the same angular momentum as the Moon! This pretty much shoots down one leading contender for Theia as a co-orbiting planet formed in one of the two stable Lagrangian points, L4 or L5, since anything formed in Earth's orbit would have next to zero angular momentum relative to Earth
Another way to look at it, is that for a Mars-sized body falling toward Earth to just graze Earth's limb with its center of gravity would require an initial tangential velocity relative to Earth of 7.08 km/s (added vectorially to the radially directed 11.2 km/s escape velocity) which couldn't be attained in Earth orbit starting from a stable L4 or L5 Lagrangian point.